At the present time when global environment problems attract people's attention, reduction of nitrogen oxides is one of important problems to be solved, and is an urgent task. In methods for reducing NOx, it is important to inhibit the NOx generation. As the method for inhibition of NOx generation, recycling of exhaust gas, lean combustion, thick and thin combustion, staged combustion, and the like can be given as examples. These methods have been widely used for industrial use or consumers. Although NOx reduction can be partially achieved by low-NOx combustors adopting these technique, a NOx reduction method, which is further effective, have been requested.
As one method for reducing NOx, which has been researched and developed from the past, there is a method (below, it is called forced vibration combustion) in which flow rate of fuel, and air which is an oxidizing agent and the like are periodically changed. Thereby, a kind of temporal thick and thin combustion is carried out (Patent Documents No. 1 to 6).
In these methods, flow rate of a fuel fluid or an oxidizing agent fluid, or flow rate of both a fuel fluid and an oxidizing agent fluid are changed to change the theoretical mixture ratio in combustion flame, and thereby making combustion in fuel rich conditions and fuel lean conditions alternatively.
In addition, Patent Document No. 7 discloses a method for reducing nitrogen oxide by using pure oxygen as an oxidizing agent, and utilizing pulsation combustion in fuel rich conditions, so called forced vibration combustion, and a device for carrying out the method.